MEASURING SOIL INVERTEBRATE DIVERSITY

Diversity and the measurement of diversity are central to many issues in ecological research as well as for applying ecology to real world problems. Every textbook in ecology devotes considerable description and explanation of species diversity, species richness, and species evenness. Community ecologists use measures of diversity to study and explain ecological patterns in many different types of communities.

In terrestrial ecosystems, litter decomposition has important effects on processes such as nutrient cycling and community structure. Decomposition is affected by the type and quality of litter, climate, the edaphic conditions (including soil temperature, hydration, and chemistry), and the community of decomposer organisms (Swift et al. 1979).

Figure - Interactions among factors that control litter decomposition (from Swift et al. 1979).

This model shows the relationships among the three factors that govern litter decomposition rates: the Biota (structure and activity of the biotic soil food webs, i.e., microbes, invertebrates, vertebrates), the Physico-chemical environment (climate, habitat, edaphic factors, i.e., contributions from the non-living environment); and Resource Attributes (primarily plant species diversity and tissue chemistry, i.e., contributions from the living environment). Many studies have shown how both the living and the non-living environments affect soil community structure and diversity (Swift et al. 1979, Ingham et al. 1982, Freckman & Virginia 1997). For example, decomposition of plant litter that is high in lignin and/or low in nutrients and is therefore difficult to decompose (resource quality) leads to dominance by fungal-feeding groups in the soil food web (namely, some taxa of nematodes, mites and Collembola), whereas easily broken-down litter is decomposed primarily by bacteria, which is reflected higher up the food chain (Coleman & Crossley 1996). And soil community diversity is at least partially determined by plant community diversity (Siemann et al. 1998). So in this case, the living environment is determining the soil community. On the other hand, recent work suggests that composition and biodiversity of soil organisms itself may have a greater effect on decomposition than has been previously recognized (González and Seastedt 2001, Wardle and Lavelle 1997, Wardle et al., 2003), especially in tropical ecosystems. So in this case, the soil biota is the driving force of the Physico-chemical environment and therefore the Resource Attributes in the Swift et al. (1979) model above. On yet a third hand, the soil Biota can directly affect the Resource Attributes. De Deyn et al. (2003) showed that soil fauna enhanced succession and diversity in a grassland community.

Soil invertebrates play important roles in soil communities. Some directly consume detritus, others consume detritivores, whereas others are higher-level carnivores that can indirectly control decomposition by their effects on lower levels of the food web (see Soil Food Web figure on the next page). The classic study of detrital food webs was conducted by Gist and Crossley (1975), showing which invertebrate groups are detritivores and which are carnivorous. This reference is somewhat hard to find; Smith and Smith (2001, p. 496) has a good description of the major findings of that study.

Soil invertebrates are clearly affecting litter decomposition rates, soil aeration, nutrient mineralization, primary production, and other ecosystem services related to soil ecosystem function and agroecological conservation (e.g., Six et al. 2002). With interest in global climate change has come the realization that soil biota may strongly affect soil CO2 sequestration and release, which is a critical variable in climate change models. Agroscientists and restoration ecologists have found that soil biota play critical roles in toxic chemical and metal mobility and remediation; they directly affect disturbed ecosystem recovery/ ecological restorations that occur after fire, UV-B exposure, post-urbanization, and herbicide-stressed soils (e.g., Lal 2002). Bioprospectors carry out the search for novel antibiotics and other drugs among the billions of soil microorganisms. Soil invertebrates are also recognized for their role in mediating or determining belowground interactions among plants. Because they are often prey for vertebrates such as birds and mammals, they have vital roles in the food chains that include those animals.

Most students of ecology rarely have an opportunity to manipulate data sets that are self-generated and then derive diversity indices and/or graphical representations of diversity. In this class you have that opportunity, and the Soil Invertebrates Diversity Laboratory is designed to enhance your skills at calculating and interpreting diversity indices.

Lab by: Boyce, Richard L. 2005. Life under your feet: Measuring soil invertebrate diversity. Teaching Issues and Experiments in Ecology, Volume 3, Ecological Society of America.

References Cited

Coleman, D. C., and D. A. Crossley, Jr. 1996. Fundamentals of soil ecology. Academic Press, San Diego.

De Deyn, G. B., C. E. Raaijmakers, H. R. Zoomer, M. P. Berg, P. C. de Ruiter, H. A. Verhoef, T. M. Bezemer, and W. H. van der Putten. 2003. Soil invertebrate fauna enhances grassland succession and diversity. Nature 422:711-713.

Freckman, D., and R. A. Virginia. 1997. Low diversity Antarctic soil nematode communities: Distribution and response to disturbance. Ecology 78:363-369.

Gist, C. S., and D. A. Crossely, Jr. 1975. A model of mineral-element cycling for an invertebrate food web in a southeastern hardwood forest litter community. In Mineral cycling in Southeast ecosystems, Pages 84-106 in J.B. Gentry and M.H. Smith (eds.). National Technical Information Service, U.S. Dept. Commerce, Washington, DC.

González, G., and T. R. Seastedt. 2001. Soil fauna and plant litter decomposition in tropical and subalpine forests. Ecology 82:955-964.

Ingham, R. E., J. A. Trofymow, R. V. Anderson, and D. C. Coleman. 1982. Relationships between soil type and soil nematodes in a shortgrass prairie. Pedobiologica 24:139-144.

Lal, R. 2002. Soil carbon sequestration in China through agricultural intensification, and restoration of degraded and desertified ecosystems. Land Degradation and Development 13:469-478.

Siemann, E., D. Tilman, J. Haarstad, and M. Ritchie. 1998. Experimental tests of the dependence of arthropod diversity on plant diversity. American Naturalist 152:738-750.

Six, J., C. Feller, K. Denef, S. M. Ogle, J. C. de Moraes Sa, and A. Albrecht. 2002. Soil organic matter, biota and aggregation in temperate and tropical soils - Effects of no-tillage. Agronomie 22:755-775.

Smith, R. L., and T. M. Smith. 2001. Ecology and field biology. 6th ed. Benjamin Cummings, San Francisco.

Swift, M. J., O. W. Heal, and J. M. Anderson. 1979. Decomposition in terrestrial ecosystems. Blackwell Scientific Publications, London.

Wardle, D. A., and P. Lavelle. 1997. Linkages between soil biota, plant litter quality and decomposition. Pages 107-124 in G. Cadisch and K. E. Giller, eds. Driven by nature: plant litter quality and decomposition. CAB International, London.

Wardle, D. A., G. W. Yeates, G. M. Barker, P. J. Bellingham, K. I. Bonner, and W. M. Williamson. 2003. Island biology and ecosystem functioning in epiphytic soil communities. Science 301: 1717-1720.